This invention relates in general to communication systems having a transmitter and a receiver, and more specifically to an apparatus and method for generating comfort noise in an open system where there is no defined protocol between the transmitter and receiver.
In asynchronous voice communication systems, it is possible to take advantage of the silence periods in a speech signal to reduce the amount of data sent from a transmitter to a receiver. For example, Discontinuous Transmission (DTX) systems are known whereby the transmitter sends a minimal amount of information during the silence periods rather than continuously transmitting the actual background noise. This Silence Insertion Descriptor (SID) information describes the spectral and level characteristics of the background noise not sent by the transmitter. The receiver uses this SID information to regenerate the background noise (this is known in the art as Comfort Noise Generation (CNG)). Many such CNG schemes have been described and implemented with success. However, all such systems require the transmitter and receiver to use a predefined protocol for exchanging the SID information (i.e. they are xe2x80x9cclosed systemsxe2x80x9d).
The following are examples of such prior art systems:
[1] ITU, G.723.1 Annex A, Silence Compression Scheme
[2] ITU, G.729 Annex B, Silence Compression Scheme
[3] ITU R M. 1073-1, Digital cellular land mobile telecommunication systems, annex 2: General description of the GSM system
[4] U.S. Pat. No. 5,960,389, Methods for generating comfort noise during discontinuous transmission
[5] U.S. Pat. No. 5,630,016, Comfort noise generation for digital communication systems
[6] U.S. Pat. No. 5,537,509, Comfort noise generation for digital communication systems
[7] U.S. Pat. No. 5,794,199, Method and System for improved discontinuous speech transmission
In the case of xe2x80x9copen systemsxe2x80x9d where there is no such protocol, the transmitter simply stops transmitting during silence periods. The receiver then enters an underrun condition. A few straightforward schemes have been implemented in prior art xe2x80x9copen systemsxe2x80x9d in order to avoid such an underrun condition during transmitter silence periods. These schemes include playing out zeros at the receiver, playing out white or coloured noise at a fixed level, as well as estimating the level of the background noise (for instance with the level of the last frame received) and playing out fixed white or coloured noise at that level. It is well know in the art that these schemes result in noticeable transitions between the background noise of the signal being transmitted and the comfort noise generated by the receiver. These artefacts greatly affect the overall speech quality. In order to achieve good speech quality, the generated comfort noise has to be of substantially the same level and spectral characteristics as the background noise of the speech signal.
According to the present invention, a Comfort Noise Generation (CNG) system is provided for use in xe2x80x9copen systemsxe2x80x9d where there is no predefined protocol for transmission of SID information from the transmitter to the receiver. As discussed above, in such systems, the transmitter simply stops transmitting during silence periods. The receiver then enters an underrun condition and generates comfort noise with the least possible impact on the overall speech quality. More particularly, according to the present invention, the computation of the level and spectral characteristics of the background of the speech signal is done within the receiver, thereby overcoming the lack of a protocol to transmit the SID information during silence periods. These characteristics are computed as a gain parameter and a set of Linear Prediction Coding (LPC) parameters which are applied to a filter which filters flat-spectrum noise in order to generate noise that sounds like the background noise of the speech signal.